In the preparation of specimens for examination by transmission electron microscopy, ion-milling machines have been employed for producing specimen thicknesses on the order of 1 micrometer. An ion-milling machine for this purpose will include at least two oppositely disposed, ion plasma guns directed towards opposing surfaces of the specimen. The force exerted by each plasma beam can thereby be balanced to prevent specimen bulging. Thin-foil specimens prepared in this manner are of value in studying transmission ion scattering or Rutherford scattering as one means of determining radiation damage in previously exposed materials.
Ordinarily, each ion-plasma gun includes an elongated first electrode with a hollow end portion and a perforated closure along with a flat, perforated second electrode. An ionizable gas is supplied at less than atmospheric pressure into the space between the electrodes and into the hollow end portion of the first electrode. The first electrode is connected to a source of high electrical potential and is insulated from the gun housing by a dielectric sleeve concentric with the electrode. In this type of arrangement the second electrode can be gounded to the gun housing.
Heretofore ion-plasma guns of this type have included a number of problems and disadvantages which rendered such milling operations tedious and difficult. Insulating and sealant material disposed within high-temperature regions have vaporized and degraded, causing carbonaceous deposits on the electrodes. Often arc initiation voltages of 2 to 3 times the operating voltage are required and are not reduced promptly enough to prevent intense degradation of these materials. Difficulties in arc initiation are aggravated when an inadequate amount of ionizable gas is provided into the space beween the electrodes. Consequently, frequent disassembly and cleaning have been necessary to maintain adequate milling characteristics.
Another and perhaps more serious difficulty has been the inability to achieve uniform specimen thickness across a milled area. Regions of variable thickness within a specimen interfere with accurate examination by electronic microscopy. Previous attempts to improve uniformity have involved rotating the specimen during the milling operation. Although this procedure has resulted in some improvement in uniformity, the elevated temperatures near the arc center have tended to produce a dish-shaped milled region.
Therefore, in view of these disadvantages within prior art, ion-plasma guns, it is an object of the present invention to provide an improved plasma gun capable of achieving uniform specimen thickness.
It is a further object to provide a plasma gun constructed to minimize degradation of insulating members and resulting electrode fouling.
It is also an object to provide a plasma gun capable of arc initiation at reduced voltages.